1. Field of the Invention
The present invention relates to an optical waveguide circuit, and particularly to an optical waveguide circuit having a loss component that causes a diffraction loss to light propagating through an optical waveguide.
2. Description of the Related Art
Recently, research and development has been carried out intensively of planar lightwave circuits (PLC) composed of silica-based glass waveguides formed on silicon substrates.
There are various types of the planar lightwave circuits. For example, H. Takahashi, et al., “Arrayed-Waveguide Grating for Wavelength Division Multi/Demultiplexer With Nanometer Resolution” (Electron. Lett., vol. 26, no. 2, pp. 87–88, 1990) discloses an optical wavelength multi/demultiplexer like an arrayed-waveguide grating (AWG); and M. Okuno et al., “8×8 Optical Matrix Switch Using Silica-Based Planer Lightwave Circuits” (IEICE Trans. Electron., vol. 76-C, no. 7, pp. 1215–1223, 1993) discloses an optical path switching like a thermo-optic (TO) switch.
Furthermore, as for a hybrid planar lightwave circuit integrating a semiconductor optical device on a planar lightwave circuit, T. Tanaka et al., “Integrated Extra Cavity laser Composed of Spot-Size Converted LD and UV Written Grating in Silica Waveguide on Si” (Electron. Lett., vol. 32, no. 13, pp. 1202–1203, 1996) discloses an external cavity frequency-stabilized laser, for example.
FIG. 1 is a plan view showing a configuration of a conventional arrayed-waveguide grating; and FIG. 2 is a cross-sectional view taken along a line II—II of FIG. 1.
In FIGS. 1 and 2, a cladding layer 202 composed of silica-based glass is formed on a silicon substrate 201, and arrayed-waveguides 203 with cores composed of silica-based glass are formed in the cladding layer 202.
At both ends of the arrayed-waveguides 203, slab waveguides 205a and 205b are connected. In addition, input waveguides 204a are connected to the input side of the slab waveguide 205a, and output waveguides 204b are connected to the output side of the slab waveguide 205b. 
FIG. 3 is a plan view showing a configuration of a conventional thermo-optic switch; and FIG. 4 is a cross-sectional view taken along a line IV—IV of FIG. 3.
In FIGS. 3 and 4, a cladding layer 212 composed of silica-based glass is formed on a silicon substrate 211, and arm waveguides 213a and 213b with cores composed of silica-based glass are formed in the cladding layer 212.
At both ends of the arm waveguides 213a and 213b, directional couplers 215 and 216 are connected. In addition, input waveguides 214a and 214b are connected to the input side of the directional coupler 215, and output waveguides 217a and 217b are connected to the output side of the directional coupler 216.
Furthermore, a thin-film heater 218 is formed on the cladding layer 212 at a position corresponding to the arm waveguide 213a, and the thin-film heater 218 is connected to wiring conductors 219a and 219b. 
FIG. 5 is a perspective view showing a configuration of a conventional external cavity frequency-stabilized laser.
In FIG. 5, a cladding layer 222 composed of silica-based glass is formed on a silicon substrate 221. In addition, an optical waveguide 223 with a core composed of silica-based glass is formed in the cladding layer 222, and a UV written grating 224 is formed in the optical waveguide 223.
Furthermore, a silicon terrace 225 is formed on the silicon substrate 221 by removing part of the cladding layer 222, and a semiconductor laser 226 is installed on the silicon terrace 225.
The optical waveguide circuits such as the planar lightwave circuits described above can improve their characteristics or carry out a new function by forming a groove by removing part of the optical waveguides or by filling the groove with a material with appropriate characteristics.
In addition, the optical waveguide circuits such as the planar lightwave circuits described above can increase flexibility in their layout by intersecting the optical waveguides, thereby enabling a variety of circuit components to be integrated on the same substrate, and implementing a new function.
Here, it is unavoidable that diffraction losses are produced in the grooves formed in part of the optical waveguides or at the intersections of the optical waveguides. As a method to reduce such diffraction losses, a technique is proposed that is increasing or decreasing the width or thickness of the waveguide, in which the groove or intersection are included, by providing a taper waveguides. In the case of increasing the width or thickness of the waveguide, the mode spot size of the lightwave magnified in accordance with the waveguide width and thickness. In the case of decreasing the width or thickness of the waveguide, the mode spot size of the lightwave also magnified, because the light confinement to the waveguide is weakened. This magnification of the mode spot size decreases a radiation angle (diffraction angle) at the groove or intersection.
International publication No. WO98/36299 discloses an example of removing part of the optical waveguides to form a groove, and of filling the groove with a material with appropriate characteristics. It removes part of the cladding and cores of an arrayed-waveguide grating to form the groove, and fills the groove with a temperature compensation material with a refractive index temperature coefficient different in sign from the temperature coefficient of the effective refractive index of the optical waveguides, thereby eliminating the temperature dependence of the transmission wavelength of the arrayed-waveguide grating. This is called an athermal arrayed-waveguide grating.
Japanese Patent Application Laid-open No. 2000-29079 discloses another example of removing part of the optical waveguides to form a groove, and of filling the groove with a material with appropriate characteristics. It removes part of the cladding and cores of a thermo-optic switch to form the groove, and fills the groove with a temperature compensation material with a refractive index temperature coefficient different in sign from the temperature coefficient of the effective refractive index of the arm waveguides, thereby reducing the electrical power consumption of the thermo-optic switch. This is called a polymer assisted thermo-optic switch.
Japanese Patent Application Laid-open No. 11-97784(1999) discloses still another example of removing part of the optical waveguide to form a groove, and of filling the groove with a material with appropriate characteristics. It removes part of the cladding and core of the waveguide between the UV written grating of the frequency-stabilized laser and the semiconductor laser, and fills the groove with a temperature compensation material with a refractive index temperature coefficient different in sign from the temperature coefficient of the refractive index of the semiconductor laser, thereby reducing the mode hopping of the frequency-stabilized laser due to temperature changes.
FIG. 6 is a plan view showing a configuration of a conventional athermal arrayed-waveguide grating; FIG. 7 is a plan view showing a single optical waveguide of FIG. 6; FIG. 8 is an enlarged plan view showing optical waveguides of FIG. 6; FIG. 9A is a cross-sectional view taken along a line IXA—IXA of FIG. 8; and FIG. 9B is a cross-sectional view taken along a line IXB—IXB of FIG. 8.
In FIGS. 6, 7, 8, 9A and 9B, a cladding layer 232 composed of silica-based glass is formed on a silicon substrate 231. In the cladding layer 232, arrayed-waveguides 236 with cores composed of silica-based glass are formed as shown in FIG. 6. In addition, linear waveguides 233 are added to the arrayed-waveguides 236. The arrayed-waveguides 236 are configured such that their lengths are increased by a fixed amount ΔL step by step toward outside.
Furthermore, a groove 237 is formed across the linear waveguides 233 by removing part of the cladding layer 232 and cores from the linear waveguides 233. Thus, focusing attention to the single optical waveguide 241 of the linear waveguides 233 as shown in FIG. 7, the optical waveguide 241 is configured such that it is divided by the groove 242 with a spacing W, and the gap is filled with a temperature compensation material 243.
Next, consider a configuration that has tapered optical waveguides inserted before and after the groove 237 to reduce the diffraction loss by the groove 237 and to increase the spacing between the optical waveguides 23.3 divided by the groove 237. FIG. 10 is a plan view showing a single optical waveguide of FIG. 6; and FIG. 11 is an enlarged plan view showing the optical waveguides of FIG. 6.
In FIG. 10, the optical waveguide 241 has a wide section 241c and narrow sections 241a and 241e. The wide section 241c is smoothly connected to the narrow sections 241a and 241e via tapered sections 241b and 241d, respectively. The groove 242 is formed such that it divides the wide section 241c. 
In FIG. 11, the optical waveguides 233 have wide sections 233c and narrow sections 233a and 233e. The wide sections 233c are smoothly connected to the narrow sections 233a and 233e via tapered sections 233b and 233d. The groove 237 is formed such that it divides the wide sections 233c. 
In addition, the groove 237 in each linear waveguide 233 is filled with a temperature compensation material 238 as shown in FIG. 9B. In particular, as for the temperature compensation material 238, it is preferable that its refractive index temperature coefficient dn′/dT differ in sign from the effective refractive index temperature coefficient dn/dT of the arrayed-waveguides 236, and that the absolute value |dn′/dT| of its refractive index temperature coefficient be sufficiently greater than the absolute value |dn/dT| of the effective refractive index temperature coefficient of the arrayed-waveguides 236. As an example of such a temperature compensation material 238, there is silicone with a refractive index temperature coefficient dn′/dT of about −40×(dn/dT).
The groove 237 in the linear waveguide 233 is formed such that its width is gradually increased by an amount ΔL′ proportional to the fixed amount ΔL which is the incremental length of the arrayed-waveguides 236 as shown in FIG. 11. In addition, the amount ΔL′ is designed to satisfy the relationship (ΔL−ΔL′)/ΔL′=−(dn′/dT)/(dn/dT) to eliminate the temperature dependence of the transmission wavelength of the arrayed-waveguide grating.
In addition, slab waveguides 235a and 235b are connected to both ends of the arrayed-waveguides 236. Input waveguides 234a are connected to the input side of the slab waveguides 235a, and output waveguides 234b are connected to the output side of the slab waveguides 235b. 
FIG. 12 is a plan view showing another configuration of the optical waveguides of FIG. 6, and FIG. 13 is a plan view showing a single optical waveguide of FIG. 12.
In FIGS. 12 and 13, a plurality of grooves 252a–252n are provided across the arrayed-waveguides 251, where n is an integer equal to or greater than two. These grooves 252a–252n are filled with temperature compensation materials 253a–253n. Focusing attention to a single optical waveguide 261 of the arrayed-waveguides 251, it has n grooves 262a–262n with widths W1, W2, . . . , and Wn, which are filled with temperature compensation materials 263a–263n as shown in FIG. 13, thereby being divided by spacings d1, d2, . . . , and dn−1.
As for a configuration which has tapered optical waveguides inserted before and after the grooves 252a–252n to reduce the diffraction loss in the grooves 252a–252n and to increase the spacings between the optical waveguides 251 divided by the grooves 252a–252n, FIG. 14 is an enlarged plan view showing the optical waveguides of FIG. 6.
In FIG. 14, the optical waveguides 251 have a wide section 251c and narrow sections 251a and 251e, and the wide section 251c is smoothly connected to the narrow sections 251a and 251e via the tapered sections 251b and 251d. The grooves 252a–252n are provided such that they divide the wide section 251c. 
In addition, as shown in FIG. 12, the grooves 252a–252n of the arrayed-waveguides 251 are lengthened by an amount ΔL′/n proportional to the fixed amount ΔL in accordance with the lengths of the arrayed-waveguides 251, which are increased by the fixed amount ΔL step by step.
FIG. 15 is a plan view showing a configuration of a conventional polymer assisted thermo-optic switch; and FIG. 16 is a cross-sectional view taken along a line XVI—XVI of FIG. 15.
In FIGS. 15 and 16, a cladding layer 272 composed of silica-based glass is formed on a silicon substrate 271, and arm waveguides 273a and 273b with cores composed of silica-based glass are formed in the cladding layer 272.
In addition, a thin-film heater 274 is formed on the cladding layer 272 in such a manner that the thin-film heater 274 is placed between the arm waveguides 273a and 273b, and is connected to the wiring conductors 275a and 275b. 
A groove 276 formed across the arm waveguide 273a by removing part of the cladding layer 272 and core. The groove 276 is filled with a temperature compensation material 277 with a refractive index temperature coefficient different in sign from the temperature coefficient of the effective refractive index of the arm waveguide 273a. As the temperature compensation material 277, silicone can be used, for example. Besides, as shown in FIG. 13, a plurality of grooves can be used instead of the single groove 276.
FIG. 17 is a plan view showing a configuration of a conventional external cavity frequency-stabilized laser whose mode hopping is supressed; FIG. 18A is a cross-sectional view taken along a line XVIIIA—XVIIIA of FIG. 17; and FIG. 18B is a cross-sectional view taken along a line XVIIIB—XVIIIB of FIG. 17. In FIGS. 17, 18A and 18B, a cladding layer 282 composed of silica-based glass is formed on the silicon substrate 281. An optical waveguide 283 with a core composed of silica-based glass is formed in the cladding layer 282, and a UV written grating 284 is formed in the optical waveguide 283.
In addition, a silicon terrace 285 is formed on the silicon substrate 281 by removing the cladding layer 282, and a semiconductor laser 286 is mounted on the silicon terrace 285.
Furthermore, a groove 287 is formed in the optical waveguide 283 by removing part of the cladding layer 282 and core. The groove 287 is filled with a temperature compensation material 288 with a refractive index temperature coefficient different in sign from the temperature coefficient of the effective refractive index of the optical waveguide 283. As the temperature compensation material 288, silicone can be used, for example. Besides, as shown in FIG. 13, a plurality of grooves can be used instead of the single groove 287.
FIG. 19 is a plan view showing a configuration of a conventional crossed optical waveguide. In FIG. 19, optical waveguides 291 and 292 whose cladding and cores are each composed of silica-based glass are placed on a silicon substrate such that they intersect with each other. The intersection angle a of the optical waveguides 291 and 292 can be determined in accordance with the layout of the entire planar lightwave circuit.
FIG. 20 is a plan view showing another configuration of a conventional temperature compensated arrayed-waveguide grating; and FIG. 21 is an enlarged plan view showing the neighborhood of a slab waveguide 303a of FIG. 20.
In FIGS. 20 and 21, arrayed-waveguides 302, slab waveguides 303a and 303b, and input and output waveguides 304a and 304b, the cores and cladding of all of which are composed of silica-based glass, are formed on a silicon substrate 301. The arrayed-waveguides 302 are configured such that their lengths each increase by the fixed amount ΔL toward outside.
In addition, a groove 305 is formed in the slab waveguide 303a in such a manner that the width of the groove 305 increases by an amount ΔL′ proportional to the fixed amount ΔL step by step as the groove is crossed by the lines connecting the input waveguide and the arrayed-waveguides 302 as shown in FIG. 21. Furthermore, the groove 305 is filled with a temperature compensation material 306 with the refractive index temperature coefficient different in sign from the temperature coefficient of the effective refractive index of the arrayed-waveguides 302. As the temperature compensation material 306, silicone can be used, for example. In addition, a plurality of grooves can be used instead of the single groove 305.
On the other hand, as for the planar lightwave circuit, S. Suzuki et al., “High-Density Integrated Planar Lightwave Circuits Using SiO2—GeO2 Waveguides with a High Refractive Index Difference” (J. Lightwave Technol., vol. 12, no. 5, pp. 790–796, 1994) discloses a technique that miniaturizes the circuit by reducing the radius of curvature of the optical waveguides by increasing the refractive index contrast of the optical waveguides.
For example, a passive planar lightwave circuit such as the arrayed-waveguide grating or thermo-optic switch described above can reduce its size by using high refractive index contrast optical waveguides.
In addition, a hybrid planar lightwave circuit integrating a semiconductor laser can reduce the coupling loss between the semiconductor laser and silica-based glass waveguide by using a high refractive index contrast optical waveguide.
Today, the total reduction in size and cost of the arrayed-waveguide grating is required. Therefore it is important not only to reduce the size by using the high refractive index contrast optical waveguides, but also to eliminate the temperature control by using the technique for athermalizing the transmission wavelength.
In addition, the reduction in size and electrical power consumption of the thermo-optic switch is also required. Thus, it is important to apply the high refractive index contrast optical waveguides to the polymer assisted thermo-optic switch described above.
As described above, it is necessary for the planar lightwave circuits to form the groove or grooves by removing part of the optical waveguides and to fill the groove or grooves with a material with an appropriate characteristics in order to eliminate the temperature dependence of the transmission wavelength of the arrayed-waveguide grating, or to reduce the electrical power consumption of the thermo-optic switch, or to suppress the mode hopping of the frequency-stabilized laser due to the temperature change.
Thus, the conventional planar lightwave circuits have a problem of increasing the diffraction loss in the groove or grooves formed in the optical waveguides. As a result, the arrayed-waveguide grating and thermo-optic switch have a problem of degrading the loss characteristics, and the frequency-stabilized laser has a problem of increasing the threshold current during oscillation.
Furthermore, the planar lightwave circuits have their optical waveguides intersect with each other to increase flexibility in the circuit layout, and to give new functions by integrating a variety of circuit components on the same substrate.
However, in the intersection of the optical waveguides in the planar lightwave circuits presents, there is a problem of degrading the loss characteristics of the planar lightwave circuits because of the diffraction loss at the intersection.
Moreover, as for the technique using the high refractive index contrast optical waveguides to miniaturize the arrayed-waveguide grating and thermo-optic switch, and to reduce the coupling loss between the semiconductor device and optical waveguide in the frequency-stabilized laser, the diffraction loss in the groove or grooves is greater when using the high refractive index contrast optical waveguides than when using ordinary refractive index contrast optical waveguides. Thus, it offers a problem of degrading the loss characteristics of the arrayed-waveguide grating or thermo-optic switch, and of further increasing the threshold current during the oscillation of the frequency-stabilized laser.
FIG. 22 is a graph illustrating the relationships between the length of the optical waveguide removed by the groove and the diffraction loss. FIG. 22 illustrates a comparison between the diffraction loss in an optical waveguide with the refractive index contrast of 0.75% and the core width×core thickness=6.0 μm×6.0 μm (solid line), and the diffraction loss in the optical waveguide with the refractive index contrast of 1.5% and the core width×core thickness=4.5 μm×4.5 μm (dotted line). It is seen from FIG. 22 that the optical waveguide with the refractive index contrast of 1.5% and the core width×core thickness=4.5 μmμ4.5 μm has the excess loss twice or more in terms of dB.
As for the technique applying the high refractive index contrast optical waveguides to the optical waveguide circuit including the intersection of the optical waveguides, its excess loss at the intersection is greater than that of the circuit using the ordinary refractive index contrast optical waveguides, thereby causing a problem of further degrading the loss characteristics of the planar lightwave circuit.
For example, consider the excess loss when the optical waveguides with the same structure intersect only once at an intersection angle of 45° In this case, although the excess loss in the optical waveguides with the refractive index contrast of 0.75% and core width×core thickness=6.0 μm×6.0 μm is 0.020 dB, that of the optical waveguides with the refractive index contrast of 1.5% and core width×core thickness=4.5 μm×4.5 μm is 0.035 dB.
As a conventional technique to reduce the diffraction loss in the groove or grooves formed in part of the optical waveguides or the diffraction loss at the intersection of the optical waveguides, there is a method of widening or narrowing the optical waveguides by providing tapered waveguides before and after the groove or intersection as described above. The method, however, has a problem of increase in the size because of the addition of the tapered waveguides. Besides, as for the planar lightwave circuits, an additional fabrication process required for forming the vertically tapered waveguides causes a problem of increase in the fabrication time and cost.
When only the horizontally tapered waveguides are installed to circumvent the foregoing problem, the reduction of the diffraction loss is halved as compared with the case where the tapered waveguides are installed in both the vertical and horizontal directions.